It is demanded in mobile telecommunication devices represented by cellular phones and base stations thereof, network-associated electronic devices such as servers and routers, and large scale computers, to transmit and process large amounts of data at a high speed with low loss. In the case of transmitting and processing large amounts of data, since electric signals are of high frequencies, rapid transmission and processing of data can be achieved. However, in principle, when the frequency of an electric signal increases, the signal is definitely prone to attenuate; that is, high frequency electric signal is prone to have weakened output power, with shorter transmission distance, and has properties that are likely to cause high loss. Therefore, in order to meet the above-mentioned requirements of low loss and high speed, it is necessary to reduce the transmission loss, particularly the transmission loss in high frequency bands, among the properties of a printed circuit board itself which is mounted in a device and performs transmission and processing of data.
To obtain printed circuit boards having low transmission loss, substrate materials utilizing fluorine-based resins having low dielectric constant and low dielectric dissipation factor have been traditionally used. However, fluorine-based resins in general have high melting temperatures and high melt viscosities, while having relative low fluidity. Thus, the resins have a problem of requiring the setting of high temperature and high pressure conditions in press molding. Moreover, fluorine-based resins also have a problem that their workability, dimensional stability, and adhesiveness to metal plating are insufficient to be used for the applications in high multilayer printed circuit boards that are used in the aforementioned telecommunication devices, network-associated electronic devices, and large scale computers.
Thus, there has been conducted research on the resin materials for printed circuit boards to cope with high frequency applications and to replace the fluorine-based resins. Among them, use of polyphenylene ether which is known as one of the resins having the most excellent dielectric properties among heat resistant polymers, is attracting more attention. However, polyphenylene ether is a thermoplastic resin having high melting temperature and high melt viscosity, as is the case of the fluorine-based resins. Therefore, for the applications in printed circuit boards, resin compositions containing polyphenylene ether and a thermosetting resin in combination have been used, in order to lower the melting point and melt viscosity, and to thereby set the temperature and pressure conditions low during press molding, or for the purpose of imparting thermal resistance to polyphenylene ether to withstand a temperature greater than the melting temperature (230 to 250° C.). For example, a resin composition using an epoxy resin in combination (see Patent Document 1), a resin composition using a bismaleimide in combination (see Patent Document 2), a resin composition using a cyanate ester in combination (see Patent Document 3), a resin composition using a styrene-butadiene copolymer, or polystyrene with triallyl cyanurate or triallyl isocyanurate in combination (see Patent Documents 4 and 5), a resin composition using polybutadiene in combination (see Patent Document 6 and Patent Document 7), a resin composition prepared by preliminarily-reacting a modified polybutadiene having a functional group such as a hydroxyl group, an epoxy group, a carboxyl group or a (meth)acryl group, with bismaleimide and/or a cyanate ester (see Patent Document 8), a resin composition using triallyl cyanurate, triallyl isocyanurate, polybutadiene or the like in combination with a polyphenylene ether which has been granted or grafted with a compound having a group having an unsaturated double bond (see Patent Document 9 and Patent Document 10), a resin composition using a reaction product of polyphenylene ether and an unsaturated carboxylic acid or unsaturated acid anhydride, and bismaleimide or the like in combination (see Patent Document 11), and the like have been proposed. According to these documents, it is disclosed that in order to improve the above-described defects of thermoplasticity while maintaining the properties of low transmission loss of polyphenylene ether, the resin obtained after curing preferably does not have many polar functional groups.    [Patent Document 1] Japanese Patent Application Laid-Open No. 58-69046    [Patent Document 2] Japanese Patent Application Laid-Open No. 56-133355    [Patent Document 3] Japanese Published Examined Application No. 61-18937    [Patent Document 4] Japanese Patent Application Laid-Open No. 61-286130    [Patent Document 5] Japanese Patent Application Laid-Open No. 3-275760    [Patent Document 6] Japanese Patent Application Laid-Open No. 62-148512    [Patent Document 7] Japanese Patent Application Laid-Open No. 59-193929    [Patent Document 8] Japanese Patent Application Laid-Open No. 58-164638    [Patent Document 9] Japanese Patent Application Laid-Open No. 2-208355    [Patent Document 10] Japanese Patent Application Laid-Open No. 6-184213    [Patent Document 11] Japanese Patent Application Laid-Open No. 6-179734